This present invention relates generally to computer memory, and more specifically to phase change memories (PCMs).
A PCM is a form of resistive non-volatile computer random-access memory (RAM) that stores data by altering the state of the matter from which the device is fabricated. Phase change materials can be manipulated into two or more different phases or states, with each phase representing a different data value. Generally, each phase exhibits different electrical properties (or different resistance values). The amorphous and crystalline (or polycrystalline) phases are typically two phases used for binary data storage (1's and 0's) since they have detectable differences in electrical resistance. Specifically, the amorphous phase has a higher resistance than the crystalline phase.
Chalcogenides are a group of materials commonly utilized as phase change material. This group of materials contains a chalcogen (Periodic Table Group 16/VIA) and another element. Selenium (Se) and tellurium (Te) are the two most common elements in the group used to produce a chalcogenide semiconductor when creating a PCM memory cell. Example chalcogenides include Ge2Sb2Te5 (germanium-antimony-tellurium or “GST”), SbTe3, and In2Se3.
Altering the phase change material's state can be achieved by heating the material to a melting point and then cooling the material to one of the possible states, or by heating an amorphous region to, or near, a crystallization temperature to convert some or all of the amorphous material to crystalline form. A current passed through the phase change material creates heat and causes the phase change material to melt. Melting and gradually cooling down the phase change material allows time for the phase change material to form the crystalline state. Melting and abruptly cooling the phase change material quenches the phase change material into the amorphous state. Heating to below the melting temperature can also be used to crystallize amorphous material without melting.